Specifications
56
3706C–MICRO–2/11
AT89LP3240/6440
11.5 Pulse Width Modulation
On the AT8 9LP3240/6440, Timer 0 and Timer 1 may be independently configured as 8-bit
asymmetrical (edge-aligned) pulse width modulators (PWM) by setting the PWM0EN or
PWM1EN bits in TCONB, respectively. In PWM Mode the generated waveform is output on the
timer's input pin, T0 or T1. Therefore, C/Tx
must be s et to “0” when in PWM mode and the T0
(P3.4) and T1 (P3.5) must be configured in an output mode. The Timer Overflow Flags and
Interrupts will continue to function while in PWM Mode and Timer 1 may still generate the baud
rate for the UART. The timer GATE function also works in PWM mode, allowing the output to be
halted by an external input. Each PWM channel has four modes selected by the mode bits in
TMOD.
An example w
aveform for Timer 0 in PWM Mode 0 is shown in Figure 11-5. TH0 acts as an 8-bit
counter while RH0 stores the 8-bit compare value. When TH0 is 00H the PWM output is
set high. When the TH0 count reaches the value stored in RH0 the PWM output is set low.
Therefore, the pulse width is proportional to the value in RH0. To prevent glitches, writes to
RH0 only take effect on the FFH to 00H overflow of TH0. S etting RH0 to 00H will keep the PWM
output low.
Figure 11-5. 8-bit As
ymmetrical Pulse Width Modulation
Table 11-4.
TCONB – Timer/Counter Control Register B
TCONB = 91H Reset Value = 0010 0100B
Not Bit Addressable
PWM1EN PWM0EN PSC12 PSC11 PSC10 PSC02 PSC01 PSC00
Bit76543210
Symbol Function
PWM1EN Configures Timer 1 for Pulse Width Modulation output on T1 (P3.5).
PWM0EN Configures Timer 0 for Pulse Width Modulation output on T0 (P3.4).
PSC12
PSC11
PSC10
Prescaler for Timer 1 Mode 0. The number of active bits in TL1 equals PSC1 + 1. After reset PSC1 = 100B which
enables 5 bit
s of TL1 for compatibility with the 13-bit Mode 0 in AT89S2051.
PSC02
PSC01
PSC00
Prescaler for Timer 0 Mode 0. The number of active bits in TL0 equals PSC0 + 1. After reset PSC0 = 100B which
enables 5 bits of TL0 for compatibility with the 13-bit Mode 0 in AT89C52.
FFH
00H
RH0
(P3.4)T0
TF0 Set
TH0
time